The work described herein consists of the development of a rapid procedure for the isolation, by cDNA cloning, of DNA fragments derived from mRNA species whose concentration in the cell can be varied by the application of an appropriate stimulus. The procedure has been used to clone DNA fragments derived from the two most dramatically estrogen inducible mRNA species found in rooster liver. Vitellogenin mRNA is one of these species while the other is a much smaller mRNA 825 nucleotides long that specifies an as yet unidentified protein of 12,000 daltons. The kinetics of accumulation of the small estrogen inducible mRNA and vitellogenin mRNA are very similar. Prior to administration of the hormone both species are present in rooster liver at a level of less than one molecule per cell. Both mRNA species accumulate and decay in concert. The small estrogen inducible mRNA species has been purified to approximately 70% homogeneity by chromatography on diazotized cellulose to which a cloned DNA fragment had been attached.